1. Field of the Invention
This invention relates to a television receiver and particularly to a television receiver having a trap circuit for reducing radio interference between television and FM radio waves.
2. Description of the Prior Art
In U.S.A., radio waves of FM broadcasting stations of channels CH 201 (carrier frequency: 88.1 MHz) to CH 220 (carrier frequency: 91.9 MHz) are assigned at frequency intervals of 200 KHz to a frequency band where a television radio wave of CH 6 (video carrier frequency: 83.25 MHz) is assigned. Therefore, there is a problem of interference to television receivers developed by this overlapped assignment of frequency of these radio waves.
The FCC (Federal Communication Commission) recommend manufactures of television receivers and video tape recorders improvement in the interference on the side of the television receivers or video tape recorders because the FCC intends to support development of FM broadcasting.
Moreover, the FCC intended the manufacturers to make it the manufacturer's duty to execute FM interference countermeasure by a law to keep a sufficient improvement in this interference.
Countermeasure of such interference has been tried. For example, Japanese patent application provisional publication No. 60-80325 discloses a technique of reducing affection due to the Interference of FM radio waves by providing a trap circuit for removing frequency component of the FM radio wave at an input antenna terminal of a tuner of a television.
However, in such prior art, if attenuation is increased, there is problem that a trap having a high accuracy of frequency is required. Further, increase in attenuation causes a problem of leaking. Therefore, a sufficient attenuation of interference components has not been obtained.
Hereinbelow will be described a prior art video detection circuit and a video amplifying circuit following the video detection circuit with reference to FIGS. 4 and 5.
FIG. 4 is a block diagram of a prior art trap circuit 3 followed by a video detection circuit. In FIG. 4, the trap circuit 3 following a video detection circuit 1 and a video signal amplifying circuit 2 comprises a matching resistor R1, a variable coil L1 for determining trapping band width, and a trap X1 of 4.5 MHz (a ceramic trap). Its output signal is supplied to a video signal processing, color signal processing, and synchronizing signal separation circuits of the following stage.
The trap circuit 3 of 4.5 MHz represents a frequency characteristic as shown in FIG. 5. FIG. 5 shows a frequency characteristic of the prior art trap circuit shown in FIG. 4. The trap circuit 8 prevents a sound signal of 4.5 MHz from entering the following stage of the video signal processing, color signal processing, and synchronizing separation circuits as shown in FIG. 5. As shown in FIG. 5, the FM radio waves CH 201 is detected at 4. 85 MHz; and the FM radio waves CH 202 is detected at 5.05 MHz when the channel CH 8 is selected. Therefore, a detection level of interference components cannot be attenuated sufficiently by the trap circuit 3 shown in FIG. 4 as mentioned above. Therefore, a beat is developed between the color signal carrier frequency 3.58 MHz and these interference components. Accordingly, beat signals of 1. 27 MHz and 1.47 MHz are developed, so that a strip pattern noise due to the interference appears in the reproduced video image.
FIG. 6 shows this relation more specifically. FIG. 6 is a chart showing frequency relation of the channel CH 6 of television radio wave and FM radio waves. As shown in FIG. 6, beat signals of 1.27 MHz, 1.47 MHz, 1.67 MHz, . . . , 3.27 MHZ are developed between the detection signal frequency components and the color carrier frequency of 3.58 MHz.